That’s right. What these machines actually detect and measure is any chemical compund that contains the methyl group in its molecular structure. There are thousands of such compounds — including quite a few which can be found on the human breath. And this machine that determines a person’s guilt or innocence will “see” all of those chemicals as alcohol — and report a falsely high “blood-alcohol” concentration (BAC).

Most breath machines used by law enforcement in DUI cases today employ a technology called infrared spectroscopy. The DUI suspect breathes through a tube connected to the machine and a breath sample is captured in a small sample chamber inside the machine. Then beams of infrared light are shot through the captured breath sample. If there are any compounds containing the methyl group, they will absorb some of this light; the more of the chemical compound in the breath sample, the more light is absorbed. The more light that is absorbed, the less that reaches sensors at the other end of the sample chamber. And the less light that is detected by the sensors, the higher the supposed “blood-alcohol” reading.

Problem: the machines are, scientifically speaking, fairly unsophisticated. They are, as scientists say, non-specific — that is, they are not capable of detecting and measuring a specific compound. More important for government work, they are relatively cheap. Rather than use more expensive filters and/or multiple filters, for example, most breathalyzers use only one or three less-costly filters. Result: these machines can only detect and measure a broad range of compounds containing the methyl group — and they then simply assume that the unknown compound within this group is ethyl alcohol.

If a person has any of these other compounds on his breath, called interferents by the scientists, he will get a falsely high breath alcohol test result. And if there are two or three such compounds on his breath, the machine will read a cumulative result: it will add them up and falsely report the total as the breath-alcohol level.

So what kinds of compounds may be on a person’s breath that can cause false BAC readings in a DUI case? In one study of eight men, 69 different compounds containing the methyl group were discovered. “Trace Composition of Human Respiratory Gas”, 30 Archives of Environmental Health290. In another study invoviing 28 subjects, researchers found that the “combined expired air comprises at least 102 various organic compounds of endogenous and exogenous origin”. “Characterization of Human Expired Air”, 15 Journal of Chromatographic Sciences 240. And Canadian scientists have discovered over 200 such compounds. “The Diagnostic Potential of Breath Analysis”, 21(1) Clinical Chemistry 5.

What are these compounds? Are there any on my breath? Well, for starters, diabetics with low blood sugar can have high levels of acetone — which is “seen” as alcohol by breathalyzers. And scientific studies have found that people on diets can have reduced blood-sugar levels, causing acetone hundreds of times higher than found in normal individuals. Frank and Flores, “The Likelihood of Acetone Interference in Breath Alcohol Measurements”, 3 Alcohol, Drugs and Driving 1. And there are many other so-called “interferents”. See, for example, “Excretion of Low-Molecular Weight Volatile Substances in Human Breath: Focus on Endogenous Ethanol”, 9 Journal of Analytical Toxicology 246.

If you are a smoker, your breathalyzer result is likely to be higher than expected. The compound acetaldehyde — reported by the breathalyzer as “alcohol” — is produced in the human body as a by-product in metabolizing consumed alcohol, and eventually passes into the lungs and breath. Researchers have discovered that levels of acetaldehyde in the lungs can be 30 times higher in smokers than in non-smokers. Result: higher BAC readings on the machine.

And then there are the industrial compounds: paint, glue, gasoline, thinners, and other compounds contain the methyl group. No, you don’t have to drink the stuff: simply absorbing it through your skin or inhaling the fumes can result in significant levels of the chemical in your body for hours or even days, depending upon the half-life of the compound. So if you’ve painted a room or been around gasoline in the last day or two, don’t breath into a breathalyzer.

Some law enforcement officials say that this is not a problem, claiming that levels of the compound would have to be at toxic levels to raise a breath test result to .08% or higher. These officials are displaying their ignorance of the science involved — specifically, of the partition ratio. This is the ratio of the compound found in the breath to that found in the blood. With ethyl alcohol, the ratio is 2100-to-1, which means that, on average, there will be 2100 units of alcohol in the blood for every unit found in the breath. These officials are using this ratio for all compounds, but every compound has its own ratio. Toluene (found in paint, glue, thinners, cleaning solvents. etc.), for example, has a partition ratio of only 7-to-1; a far greater amount of toluene in the blood will pass into the breath, and so a much smaller amount in the body will have a far greater impact on the breath machine.

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